The transforming growth factor-? (TGF-?) superfamily encompasses a large group of secreted signaling molecules that play critical roles in regulating the development of many different tissues during embryogenesis as well as in maintaining and modulating the functions of these tissues in adult animals. Much of our work has focused on two highly related family members, myostatin (GDF-8) and GDF-11, which we originally cloned on the basis of their homology to known family members. We showed that mice engineered to lack myostatin have dramatic increases in skeletal muscle mass throughout the body resulting from a combination of muscle fiber hyperplasia and hypertrophy, demonstrating that myostatin normally acts to limit muscle mass. We also showed that loss of myostatin has significant beneficial effects on fat and glucose metabolism, including in models of obesity and type II diabetes. In the case of GDF-11, we showed that Gdf11 knockout mice have defects in patterning of the axial skeleton and in kidney development. Others have demonstrated that GDF-11 also plays important roles in the development of the nervous system and pancreas. Much of our effort in the past has focused on understanding the developmental consequences of eliminating the functions of these molecules as well as on identifying key regulatory and signaling components. We would now like to focus our effort on better understanding their postnatal roles, with the long-term goal of exploiting their activities for clinical applications, particularly in disease states affecting key metabolic tissues like skeletal muscle and adipose tissue. Critical to this process will be a thorough understanding of the cellular targets for these signaling molecules as well as their mode of action. As a starting point, we will attempt to use genetic strategies in mice to identify the cellular targets for myostatin and the ligands that cooperate with myostatin to regulate skeletal muscle growth. The Specific Aims of this project are: to determine the effect of blocking Smad function in myofibers in vivo and to determine the role of satellite cells in mediating the effects of myostatin in vivo. Taken together, these studies should provide key insights into how these signaling molecules control skeletal muscle growth. Based on what is known about the biological functions of myostatin and related ligands, there has been an intense focus on manipulating this pathway to increase muscle mass and strength in patients with muscle degenerative and wasting diseases, like muscular dystrophy, sarcopenia, and cachexia, which is often seen in patients with diseases like cancer, AIDS, and sepsis. Moreover, we believe that an equally important application for such agents may turn out to be to treat metabolic diseases, like obesity and type II diabetes, which have reached near epidemic proportions not only in adults but also in children and adolescents. We believe that the studies proposed here will be a crucial step to elucidating the functions of these molecules and developing strategies to exploiting their activities for therapeutic intervention.